Poly(isobutylene-co-isoprene), or IIR, is a synthetic elastomer commonly known as butyl rubber which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (1-2 mole %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
The first major application of IIR was in tire inner tubes. Despite the low levels of backbone unsaturation (ca. 0.8-1.8 mol %), IIR possesses sufficient vulcanization activity for inner tube application. With the evolution of the tire inner liner, it became necessary to enhance the cure reactivity of IIR to levels typically found for conventional diene-based elastomers such as butadiene rubber (BR) or styrene-butadiene rubber (SBR). To this end, halogenated grades of butyl rubber were developed.
Halobutyl rubbers are prepared through post halogenation of butyl rubber dissolved in an organic solution. For example, the treatment of solutions of IIR dissolved in hexane with elemental chlorine or bromine results in the formation of chlorobutyl (CIIR) and bromobutyl (BIIR) rubber. These halobutyl rubbers are marked by the presence of reactive allylic halides along the polymer main chain. The enhanced reactivity of these moieties (c.f. traditional elastomer unsaturation) elevates the cure reactivity of CIIR and BIIR to levels comparable to those possessed by materials such as BR and SBR. This allows for acceptable levels of adhesion between, for example, a BIIR based inner liner formulation and a BR based carcass compound. Not surprisingly, the enhanced polarizability of Br compared to Cl results in BIIR being far more reactive than CIIR. As such, BIIR is the most commercially significant grade of halobutyl rubber.
This conventional process of producing halobutyl rubber has a number of problems. The butyl rubber must first be produced, typically at temperatures from −70 to −100° C., then separated from the polymerization diluent (typically methyl chloride), dried, and then re-dissolved in a hexane solution for treatment with elemental chlorine or bromine at temperatures from 40 to 65° C. There are significant energy and solvent costs associated with this multi-step process. Furthermore, the halogenation process involves an aqueous quenching step that generates a significant volume of acid requiring neutralization before disposal. The conventional process is costly and involves multiple steps; in order to simplify the process, it would be desirable to produce a halogenated butyl rubber directly during polymerization in a single-step process by co-polymerizing an isoolefin with a halogenated co-monomer.
There have been previous attempts to co-polymerize isoolefins with halogenated co-monomers. In particular, co-polymerization with brominated co-monomers was attempted using p-Bromostyrene (Z. A. Sadykhov, F. M. Aliev, Azerb. Khim. Zh. 1970, 3, 96) and 2-Bromo-2-methyl-1,3-butadiene (EP 0 609 737). These attempts have all met with limited commercial success. However, there have been no reports in the literature on the copolymerization of Isobutylene (IB) with 4-Bromo-3-methyl-1-butene (BMB) and this particular co-monomer therefore remains unexplored.
The need therefore still exists for co-polymers of isoolefins with halogenated co-monomers, particularly brominated co-monomers, and simplified processes for producing those co-polymers.